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Fluid saturation estimation using Full Waveform Inversion (FWI): a controlled laboratory experiment
Alsaad, Ali
Alsaad, Ali
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2024
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Abstract
This thesis presents a comprehensive study on the application of Full Waveform Inversion (FWI) and fluid substitution analysis for estimating fluid saturation in a controlled laboratory setting. Monitoring fluid injection, essential for processes such as carbon capture and storage (CCS), enhanced oil recovery, and hydraulic fracturing, is crucial in minimizing environmental and operational risks. Through a novel approach, this study applies time-lapse FWI (4D FWI) in a lab experiment to detect changes in fluid saturation within a rock sample, aiming to evaluate the effectiveness of traditional methods like Gassmann fluid substitution in capturing the complexities of fluid-rock interactions under partially saturated conditions.
The methodology involves a detailed experimental setup for acoustic data acquisition on a Berea Sandstone sample, utilizing the 3D Acoustic Acquisition & Imaging System (WILD) for both baseline and monitor surveys. The study carefully outlines the process of mesh creation, data preprocessing, and the application of FWI to derive high-resolution P-wave velocity models. These models are then used to assess the validity of Gassmann fluid substitution in predicting fluid saturation changes, revealing discrepancies that suggest the need for enhanced models or additional considerations in fluid substitution analysis.
Significant findings from the research include the observation of unexpected P-wave velocity reductions in partially saturated rocks after brine injection, challenging the conventional expectations based on Gassmann’s theory. This anomaly is attributed to attenuation and dispersion due to various factors, including patchy saturation, wave-induced fluid flow (WIFF), and changes in surface energy, underscoring the complex nature of fluid-rock interactions. The study also emphasizes the importance of careful sample selection, mesh optimization, and preprocessing techniques in improving the accuracy and reliability of FWI outcomes.
The thesis concludes with valuable insights and recommendations for future research, emphasizing the need for continuous refinement of FWI parameters, creating more accurate fluid substitution models, and incorporating advanced computational and data collection methods. This research marks a significant advancement in the application of 4D FWI in laboratory experiments, offering a new perspective on fluid injection monitoring and the potential for improved reservoir characterization and management.
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